Linear Compressor

ABSTRACT

The present invention discloses a linear compressor in which a piston is driven by a linear motor and linearly reciprocated inside a cylinder to suck, compress and discharge refrigerants. The linear compressor synchronizes an operation frequency of the linear motor with a natural frequency of the piston, considering that an elastic force of a mechanical spring and a gas spring which elastically support the piston in the motion direction is varied by load. Even if the load is varied, the linear motor is operated in the resonance state, to maximize efficiency. The linear compressor varies a stroke of the piston according to the load, thereby actively handling and rapidly overcoming the load and reducing power consumption.

TECHNICAL FIELD

The present invention relates to a linear compressor which can actively handle load and efficiently perform an operation, by synchronizing an operation frequency with a natural frequency of a movable member varied by the load.

BACKGROUND ART

In general, a compressor that is a mechanical apparatus for increasing a pressure, by receiving power from a power unit system such as an electric motor or turbine and compressing air, refrigerants or other various operation gases has been widely used for home appliances such as a refrigerator and an air conditioner or in the whole industrial fields.

The compressors are roughly divided into a reciprocating compressor having a compression space through which operation gases are sucked or discharged between a piston and a cylinder, so that the piston can be linearly reciprocated inside the cylinder to compress refrigerants, a rotary compressor having a compression space through which operation gases are sucked or discharged between an eccentrically-rotated roller and a cylinder, so that the roller can be eccentrically rotated on the inner walls of the cylinder to compress refrigerants, and a scroll compressor having a compression space through which operation gases are sucked or discharged between an orbiting scroll and a fixed scroll, so that the orbiting scroll can be rotated with the fixed scroll to compress refrigerants.

Recently, among the reciprocating compressors, a linear compressor has been mass-produced because it has high compression efficiency and simple structure by removing mechanical loss by motion conversion by directly connecting a piston to a driving motor performing linear reciprocation.

Generally, the linear compressor which sucks, compresses and discharges refrigerants by using a linear driving force of the motor includes a compression unit consisting of a cylinder and a piston for compressing refrigerant gases, and a driving unit consisting of a linear motor for supplying a driving force to the compression unit.

In detail, in the linear compressor, the cylinder is fixedly installed in a closed vessel, and the piston is installed in the cylinder to perform linear reciprocation. When the piston is linearly reciprocated inside the cylinder, refrigerants are sucked into a compression space in the cylinder, compressed and discharged. A suction valve assembly and a discharge valve assembly are installed in the compression space, for controlling suction and discharge of the refrigerants according to the inside pressure of the compression space.

In addition, the linear motor for generating a linear motion force to the piston is installed to be connected to the piston. An inner stator and an outer stator formed by stacking a plurality of laminations at the periphery of the cylinder in the circumferential direction are installed on the linear motor with a predetermined gap. A coil is coiled inside the inner stator or the outer stator, and a permanent magnet is installed at the gap between the inner stator and the outer stator to be connected to the piston.

Here, the permanent magnet is installed to be movable in the motion direction of the piston, and linearly reciprocated in the motion direction of the piston by an electromagnetic force generated when a current flows through the coil. Normally, the linear motor is operated at a constant operation frequency f_(c), and the piston is linearly reciprocated by a predetermined stroke S.

On the other hand, various springs are installed to elastically support the piston in the motion direction even though the piston is linearly reciprocated by the linear motor. In detail, a coil spring which is a kind of mechanical spring is installed to be elastically supported by the closed vessel and the cylinder in the motion direction of the piston. Also, the refrigerants sucked into the compression space serve as a gas spring.

The coil spring has a constant mechanical spring constant K_(m), and the gas spring has a gas spring constant K_(g) varied by load. A natural frequency f_(n) of the piston (or linear compressor) is calculated in consideration of the mechanical spring constant K_(m) and the gas spring constant K_(g).

The thusly-calculated natural frequency f_(n) of the piston determines the operation frequency f_(c) of the linear motor. The linear motor improves efficiency by equalizing its operation frequency f_(c) to the natural frequency f_(n) of the piston, namely, operating in the resonance state.

Accordingly, in the linear compressor, when a current is applied to the linear motor, the current flows through the coil to generate an electromagnetic force by interactions with the outer stator and the inner stator, and the permanent magnet and the piston connected to the permanent magnet are linearly reciprocated by the electromagnetic force.

Here, the linear motor is operated at the constant operation frequency f_(c). The operation frequency f_(c) of the linear motor is equalized to the natural frequency f_(n) of the piston, so that the linear motor can be operated in the resonance state to maximize efficiency.

As described above, when the piston is linearly reciprocated inside the cylinder, the inside pressure of the compression space is changed. The refrigerants are sucked into the compression space, compressed and discharged according to changes of the inside pressure of the compression space.

The linear compressor is formed to be operated at the operation frequency f_(c) identical to the natural frequency f_(n) of the piston calculated by the mechanical spring constant K_(m) of the coil spring and the gas spring constant K_(g) of the gas spring under the load considered in the linear motor at the time of design. Therefore, the linear motor is operated in the resonance state merely under the load considered on design, to improve efficiency.

However, since the actual load of the linear compressor is varied, the gas spring constant K_(g) of the gas spring and the natural frequency f_(n) of the piston calculated by the gas spring constant K_(g) are changed.

In detail, as illustrated in FIG. 1A, the operation frequency f_(c) of the linear motor is determined to be identical to the natural frequency f_(n) of the piston in a middle load area at the time of design. Even if the load is varied, the linear motor is operated at the constant operation frequency f_(c). But, as the load increases, the natural frequency f_(n) of the piston increases.

$\begin{matrix} {f_{n} = {\frac{1}{2\pi}\sqrt{\frac{K_{m} + K_{g}}{M}}}} & {{Formula}\mspace{14mu} 1} \end{matrix}$

Here, f_(n) represents the natural frequency of the piston, K_(m) and K_(g) represent the mechanical spring constant and the gas spring constant, respectively, and M represents a mass of the piston.

Generally, since the gas spring constant K_(g) has a small ratio in the total spring constant K_(t), the gas spring constant K_(g) is ignored or set to be a constant value. The mass M of the piston and the mechanical spring constant K_(m) are also set to be constant values. Therefore, the natural frequency f_(n) of the piston is calculated as a constant value by the above Formula 1.

However, the more the actual load increases, the more the pressure and temperature of the refrigerants in the restricted space increase. Accordingly, an elastic force of the gas spring itself increases, to increase the gas spring constant K_(g). Also, the natural frequency f_(n) of the piston calculated in proportion to the gas spring constant K_(g) increases.

Referring to FIGS. 1A and 1B, the operation frequency f_(c) of the linear motor and the natural frequency f_(n) of the piston are identical in the middle load area, so that the piston can be operated to reach a top dead center (TDC), thereby stably performing compression. In addition, the linear motor is operated in the resonance state, to maximize efficiency of the linear compressor.

However, the natural frequency f_(n) of the piston gets smaller than the operation frequency f_(c) of the linear motor in a low load area, and thus the piston is transferred over the TDC, to apply an excessive compression force. Moreover, the piston and the cylinder are abraded by friction. Since the linear motor is not operated in the resonance state, efficiency of the linear compressor is reduced.

In addition, the natural frequency f_(n) of the piston becomes larger than the operation frequency f_(c) of the linear motor in a high load area, and thus the piston does not reach the TDC, to reduce the compression force. The linear motor is not operated in the resonance state, thereby decreasing efficiency of the linear compressor.

As a result, in the conventional linear compressor, when the load is varied, the natural frequency f_(n) of the piston is varied, but the operation frequency f_(c) of the linear motor is constant. Therefore, the linear motor is not operated in the resonance state, which results in low efficiency. Furthermore, the linear compressor cannot actively handle and rapidly overcome the load.

On the other hand, in order to actively handle and rapidly overcome the load, the conventional linear compressor varies the operation frequency f_(c) of the linear motor by controlling an input current in proportion to the load. Especially, the linear compressor controls the operation frequency f_(c) of the linear motor to be more lowered in the low load area. Thus, compression is not performed in the resonance state, which seriously reduces efficiency of the linear compressor. Nevertheless, because efficiency of the whole refrigeration cycle increases, the whole efficiency is not much changed.

In order to perform compression in the resonance state even in the low load area, the conventional linear compressor is intended to be operated in the low frequency area so that the operation frequency f_(c) of the linear motor can be equalized to the natural frequency f_(n) of the piston. However, in the linear compressor having the large mechanical spring constant K_(m), it is difficult to control the operation frequency f_(c) of the linear motor to the low frequency by adjusting the input current. Furthermore, the linear compressor cannot efficiently vary the compression capacity.

DISCLOSURE OF THE INVENTION

The present invention is achieved to solve the above problems. An object of the present invention is to provide a linear compressor which can be operated in the resonance state regardless of variations of load, by synchronizing an operation frequency of a linear motor with a natural frequency of a piston, even if the natural frequency of the piston is varied by the load.

Another object of the present invention is to provide a linear compressor which can efficiently vary a compression capacity, by enabling a linear motor to simultaneously or individually vary an operation frequency by load and control a stroke of a piston.

In order to achieve the above-described objects of the invention, there is provided a linear compressor, including: a fixed member having a compression space inside; a movable member linearly reciprocated in the fixed member in the axial direction, for sucking refrigerants into the compression space and compressing the refrigerants; one or more springs installed to elastically support the movable member in the motion direction of the movable member, spring constants of which being varied by load; and a linear motor installed to be connected to the movable member, for linearly reciprocating the movable member in the axial direction, and synchronizing its operation frequency with a natural frequency of the movable member.

Preferably, the spring constants of the springs are varied in proportion to the load, and the operation frequency of the linear motor is varied in proportion to the load.

Preferably, the linear compressor is installed in a refrigeration/air conditioning cycle, and the load is calculated in proportion to a difference between a pressure of condensing refrigerants (condensing pressure) and a pressure of evaporating refrigerants (evaporating pressure) in the refrigeration/air conditioning cycle. More preferably, the load is additionally calculated in proportion to a pressure that is an average of the condensing pressure and the evaporating pressure (average pressure).

Preferably, the springs include a mechanical spring being installed to support the movable member at both sides of the motion direction of the movable member, and having a constant mechanical spring constant, and a gas spring having a gas spring constant varied by the load of the refrigerants sucked into the compression space.

Preferably, the mechanical spring and the gas spring are formed so that the ratio of the mechanical spring constant to the total spring constant obtained by adding up the mechanical spring constant and the gas spring constant can be below 90%, and the mechanical spring constant and the gas spring constant are determined so that the natural frequency of the movable member can be set in a low frequency area between 30 and 55 Hz.

Preferably, the mechanical spring constant and the gas spring constant of the mechanical spring and the gas spring are set so that a stroke that is a linear reciprocation distance of the movable member can be varied by the load. More preferably, the mechanical spring constant and the gas spring constant of the mechanical spring and the gas spring are set so that the movable member can be linearly reciprocated to reach a top dead center even if the stroke of the movable member is varied.

Preferably, an initial position of the movable member is closer to the top dead center according to decrease of the mechanical spring constant, so that the movable member can be stably elastically supported by the mechanical spring and the gas spring.

According to another aspect of the present invention, a linear compressor includes: a fixed member having a compression space inside; a movable member linearly reciprocated in the fixed member in the axial direction, for compressing refrigerants sucked into the compression space; a mechanical spring being installed to elastically support the movable member at both sides of the motion direction of the movable member, and having a constant mechanical spring constant; a gas spring having a gas spring constant varied by load of the refrigerants sucked into the compression space; and a linear motor installed to be connected to the movable member, for linearly reciprocating the movable member in the axial direction, wherein the mechanical spring constant and the gas spring constant of the mechanical spring and the gas spring are set so that a stroke that is a linear reciprocation distance of the movable member can be varied by the load.

Preferably, the linear compressor is installed in a refrigeration/air conditioning cycle, and the load is calculated in proportion to a difference between a pressure of condensing refrigerants (condensing pressure) and a pressure of evaporating refrigerants (evaporating pressure) in the refrigeration/air conditioning cycle. More preferably, the load is additionally calculated in proportion to a pressure that is an average of the condensing pressure and the evaporating pressure (average pressure).

Preferably, the mechanical spring constant and the gas spring constant of the mechanical spring and the gas spring are set so that the movable member can be linearly reciprocated to reach a top dead center even if the stroke of the movable member is varied.

Preferably, an initial position of the movable member is closer to the top dead center according to decrease of the mechanical spring constant, so that the movable member can be stably elastically supported by the mechanical spring and the gas spring.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will become better understood with reference to the accompanying drawings which are given only by way of illustration and thus are not limitative of the present invention, wherein:

FIG. 1A is a graph showing a stroke by load in a conventional linear compressor;

FIG. 1B is a graph showing efficiency by the load in the conventional linear compressor;

FIG. 2 is a cross-sectional view illustrating a linear compressor in accordance with the present invention;

FIG. 3A is a graph showing a stroke by load in the linear compressor in accordance with the present invention;

FIG. 3B is a graph showing efficiency by the load in the linear compressor in accordance with the present invention;

FIG. 4 is a graph showing changes of a gas spring constant by the load in the linear compressor in accordance with the present invention;

FIG. 5 is a graph showing changes of the gas spring constant by variations of an ambient temperature, a mass of a piston, a mechanical spring constant and a natural frequency in the linear compressor in accordance with the present invention;

FIG. 6 is a structure view illustrating the stroke by the load in part of the linear compressor in accordance with the present invention; and

FIGS. 7A to 7C are side-sectional views illustrating an operation state of the linear compressor in accordance with the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

A linear compressor in accordance with preferred embodiments of the present invention will now be described in detail with reference to the accompanying drawings.

As shown in FIG. 2, in the linear compressor, an inlet tube 2 a and an outlet tube 2 b through which refrigerants are sucked and discharged are installed at one side of a closed vessel 2, a cylinder 4 is fixedly installed inside the closed vessel 2, a piston 6 is installed inside the cylinder 4 to be linearly reciprocated to compress the refrigerants sucked into a compression space P in the cylinder 4, and various springs are installed to be elastically supported in the motion direction of the piston 6. Here, the piston 6 is connected to a linear motor 10 for generating a linear reciprocation driving force. As depicted in FIGS. 3A and 3B, even if a natural frequency f_(n) of the piston 6 is varied by load, the linear motor 10 controls its operation frequency f_(c) to be synchronized with the natural frequency f_(n) of the piston 6, so that the resonance operation can be performed in the whole load areas to improve compression efficiency.

In addition, a suction valve 22 is installed at one end of the piston 6 contacting the compression space P, and a discharge valve assembly 24 is installed at one end of the cylinder 4 contacting the compression space P. The suction valve 22 and the discharge valve assembly 24 are automatically controlled to be opened or closed according to the inside pressure of the compression space P, respectively.

The top and bottom shells of the closed vessel 2 are coupled to hermetically seal the closed vessel 2. The inlet tube 2 a through which the refrigerants are sucked and the outlet tube 2 b through which the refrigerants are discharged are installed at one side of the closed vessel 2. The piston 6 is installed inside the cylinder 4 to be elastically supported in the motion direction to perform the linear reciprocation. The linear motor 10 is connected to a frame 18 outside the cylinder 4. The cylinder 4, the piston 6 and the linear motor 10 compose an assembly. The assembly is installed on the inside bottom surface of the closed vessel 2 to be elastically supported by a support spring 29.

The inside bottom surface of the closed vessel 2 contains oil, an oil supply device 30 for pumping the oil is installed at the lower end of the assembly, and an oil supply tube 18 a for supplying the oil between the piston 6 and the cylinder 4 is formed inside the frame 18 at the lower side of the assembly. Accordingly, the oil supply device 30 is operated by vibrations generated by the linear reciprocation of the piston 6, for pumping the oil, and the oil is supplied to the gap between the piston 6 and the cylinder 4 along the oil supply tube 18 a, for cooling and lubrication.

The cylinder 4 is formed in a hollow shape so that the piston 6 can perform the linear reciprocation, and has the compression space P at its one side. Preferably, the cylinder 4 is installed on the same straight line with the inlet tube 2 a in a state where one end of the cylinder 4 is adjacent to the inside portion of the inlet tube 2 a.

The piston 6 is installed inside one end of the cylinder 4 adjacent to the inlet tube 2 a to perform linear reciprocation, and the discharge valve assembly 24 is installed at one end of the cylinder 4 in the opposite direction to the inlet tube 2 a.

Here, the discharge valve assembly 24 includes a discharge cover 24 a for forming a predetermined discharge space at one end of the cylinder 4, a discharge valve 24 b for opening or closing one end of the cylinder 4 near the compression space P, and a valve spring 24 c which is a kind of coil spring for applying an elastic force between the discharge cover 24 a and the discharge valve 24 b in the axial is direction. An O-ring R is inserted onto the inside circumferential surface of one end of the cylinder 4, so that the discharge valve 24 a can be closely adhered to one end of the cylinder 4.

An indented loop pipe 28 is installed between one side of the discharge cover 24 a and the outlet tube 2 b, for guiding the compressed refrigerants to be externally discharged, and preventing vibrations generated by interactions of the cylinder 4, the piston 6 and the linear motor 10 from being applied to the whole closed vessel 2.

Therefore, when the piston 6 is linearly reciprocated inside the cylinder 4, if the pressure of the compression space P is over a predetermined discharge pressure, the valve spring 24 c is compressed to open the discharge valve 24 b, and the refrigerants are discharged from the compression space P, and then externally discharged along the loop pipe 28 and the outlet tube 2 b.

A refrigerant passage 6 a through which the refrigerants supplied from the inlet tube 2 a flows is formed at the center of the piston 6. The linear motor 10 is directly connected to one end of the piston 6 adjacent to the inlet tube 2 a by a connection member 17, and the suction valve 22 is installed at one end of the piston 6 in the opposite direction to the inlet tube 2 a. The piston 6 is elastically supported in the motion direction by various springs.

The suction valve 22 is formed in a thin plate shape. The center of the suction valve 22 is partially cut to open or close the refrigerant passage 6 a of the piston 6, and one side of the suction valve 22 is fixed to one end of the piston 6 a by screws.

Accordingly, when the piston 6 is linearly reciprocated inside the cylinder 4, if the pressure of the compression space P is below a predetermined suction pressure lower than the discharge pressure, the suction valve 22 is opened so that the refrigerants can be sucked into the compression space P, and if the pressure of the compression space P is over the predetermined suction pressure, the refrigerants of the compression space P are compressed in the close state of the suction valve 22.

Especially, the piston 6 is installed to be elastically supported in the motion direction. In detail, a piston flange 6 b protruded in the radial direction from one end of the piston 6 adjacent to the inlet tube 2 a is elastically supported in the motion direction of the piston 6 by mechanical springs 8 a and 8 b such as coil springs. The refrigerants included in the compression space P in the opposite direction to the inlet tube 2 a are operated as gas spring due to an elastic force, thereby elastically supporting the piston 6.

Here, the mechanical springs 8 a and 8 b have constant mechanical spring constants K_(m) regardless of the load, and are preferably installed side by side with a support frame 26 fixed to the linear motor 10 and the cylinder 4 in the axial direction from the piston flange 6 b. Also, preferably, the mechanical spring 8 a supported by the support frame 26 and the mechanical spring 8 a installed on the cylinder 4 have the same mechanical spring constant K_(m).

However, the gas spring has a gas spring constant K_(g) varied by the load. When an ambient temperature rises, the pressure of the refrigerants increases, and thus the elastic force of the gases in the compression space P increases. As a result, the more the load increases, the higher the gas spring constant K_(g) of the gas spring is.

While the mechanical spring constant K_(m) is constant, the gas spring constant K_(g) is varied by the load. Therefore, the total spring constant is also varied by the load, and the natural frequency f_(n) of the piston 6 is varied by the gas spring constant K_(g) in the above Formula 1.

Even if the load is varied, the mechanical spring constant K_(m) and the mass M of the piston 6 are constant, but the gas spring constant K_(g) is varied. Thus, the natural frequency f_(n) of the piston 6 is remarkably influenced by the gas spring constant K_(g) varied by the load. In the case that the algorithm of varying the natural frequency f_(n) of the piston 6 by the load is obtained and the operation frequency f_(c) of the linear motor 10 is synchronized with the natural frequency f_(n) of the piston 6, efficiency of the linear compressor can be improved and the load can be rapidly overcome.

The load can be measured in various ways. Since the linear compressor is installed in a refrigeration/air conditioning cycle for compressing, condensing, expanding and evaporating refrigerants, the load can be defined as a difference between a condensing pressure which is a pressure of condensing refrigerants and an evaporating pressure which is a pressure of evaporating refrigerants. In order to improve accuracy, the load is determined in consideration of an average pressure of the condensing pressure and the evaporating pressure.

That is, the load is calculated in proportion to the difference between the condensing pressure and the evaporating pressure and the average pressure. The more the load increases, the higher the gas spring constant K_(g) is. For example, if the difference between the condensing pressure and the evaporating pressure increases, the load increases. Even though the difference between the condensing pressure and the evaporating pressure is not changed, if the average pressure increases, the load increases. The gas spring constant K_(g) increases according to the load.

As illustrated in FIG. 4, a condensing temperature proportional to the condensing pressure and an evaporating temperature proportional to the evaporating pressure are measured, and the load is calculated in proportion to a difference between the condensing temperature and the evaporating temperature and an average temperature.

In detail, the mechanical spring constant K_(m) and the gas spring constant K_(g) can be determined by various experiments. Referring to FIG. 5, when the mechanical spring constant K_(m) decreases, the ratio of the gas spring constant K_(g) to the total spring constant K_(T) obtained by adding up the mechanical spring constant K_(m) and the gas spring constant K_(g) increases. In addition, the higher the ambient temperature is, namely, the more the load increases, the higher the ratio of the gas spring constant K_(g) to the total spring constant K_(T) is. When the ratio of the gas spring constant K_(g) to the total spring constant K_(T) increases, the natural frequency f_(n) is remarkably changed.

Preferably, the ratio of the gas spring constant K_(g) to the total spring constant K_(T) is set below 90%.

For example, when the ratio of the gas spring constant K_(g) to the total spring constant K_(T) exceeds 10% by setting the mechanical spring constant K_(m) below 35.5 kN/m, the natural frequency f_(n) is remarkably varied due to changes of the ambient temperature. Therefore, the operation frequency f_(c) of the linear motor 10 is easily controlled, so that the linear motor 10 can be operated in the resonance state. Moreover, the load is rapidly overcome, to reduce power consumption.

However, when the ratio of the gas spring constant K_(g) to the total spring constant K_(T) becomes lower than 10% by setting the mechanical spring constant K_(m) over 35.5 kN/m, the natural frequency f_(n) is rarely varied by changes of the ambient temperature. Accordingly, the operation frequency f_(c) of the linear motor 10 is not easily controlled, so that the linear motor 10 cannot be operated in the resonance state.

As described above, when the ratio of the gas spring constant K_(g) to the total spring constant K_(T) is high, the natural frequency f_(n) of the piston 6 is remarkably varied by changes of the load, and the operation frequency f_(c) of the linear motor 10 is easily synchronized with the natural frequency f_(n) of the piston 6. Thus, the linear motor 10 is operated in the resonance state, thereby maximizing efficiency. Furthermore, even if the operation frequency f_(c) of the linear motor 10 is operated in the low frequency area, the load can be rapidly overcome by high efficiency, which results in low power consumption.

Accordingly, the natural frequency f_(n) of the piston 6 is determined at the time of design by the mechanical spring constant K_(m), the gas spring constant K_(g) and the mass M of the piston 6. If the natural frequency f_(n) of the piston 6 is set in the low frequency area ranging from 30 to 55 Hz, which is lower than the general natural frequency f_(n) of the piston 6, the linear compressor can be efficiently operated, rapidly overcoming the load.

Especially, when the linear compressor is designed, the mechanical spring constant K_(m) is set relatively small, and the ratio of the gas spring constant K_(g) to the total spring constant K_(T) is set high. As a result, the operation frequency f_(c) of the linear motor 10 is equalized to the natural frequency f_(n) of the piston 6 even in the low load, so that the linear motor 10 can be operated in the resonance state to improve efficiency of the linear compressor. Since the linear motor 10 is operated in the low frequency area, efficiency of the whole refrigeration cycle can be improved.

The linear motor 10 includes an inner stator 12 formed by stacking a plurality of laminations 12 a in the circumferential direction, and fixedly installed outside the cylinder 4 by the frame 18, an outer stator 14 formed by stacking a plurality of laminations 14 b at the periphery of a coil wound body 14 a in the circumferential direction, and installed outside the cylinder 4 by the frame 18 with a predetermined gap from the inner stator 12, and a permanent magnet 16 positioned at the gap between the inner stator 12 and the outer stator 14, and connected to the piston 6 by the connection member 17. Here, the coil wound body 14 a can be fixedly installed outside the inner stator 12.

In the linear motor 10, when a current is applied to the coil wound body 14 a to generate an electromagnetic force, the permanent magnet 16 is linearly reciprocated by interactions between the electromagnetic force and the permanent magnet 16, and the piston 6 connected to the permanent magnet 16 is linearly reciprocated inside the cylinder 4.

When the current is applied, the linear motor 10 can vary the compression capacity by changing the operation frequency f_(c). In addition, as shown in FIG. 6, the linear motor 10 can vary the compression capacity by changing a stroke S which is a linear reciprocation distance of the piston 6 into first and second strokes S1 and S2 according to the load, by adjusting the externally-inputted current.

While linearly reciprocated inside the cylinder 4, the piston 6 forms the compression space P. Preferably, even though the stroke S of the piston 6 is varied, the piston 6 is linearly reciprocated to a point in which the piston 6 is completely compressed in the cylinder 4 not to form the compression space P, namely, a top dead center (TDC), to prevent compression efficiency from being reduced by the short stroke S.

Here, the linear motor 10 can increase both the operation frequency f_(c) and the stroke S of the piston 6 or only the stroke S of the piston 6 according to increase of the load.

However, when the load increases in the linear compressor, the gas spring constant K_(g) increases to increase the elastic force of the gas spring, and thus the stroke S of the piston 6 is more reduced than when the load is small. Therefore, the operation of the linear motor 10 must be controlled in consideration of the mechanical spring constant K_(m) and the gas spring constant K_(g) reflecting this fact.

At an initial stage, the piston 6 is installed to be separated from the TDC at a predetermined interval. When the linear compressor is designed to increase the ratio of the gas spring constant K_(g) to the total spring constant K_(T) by decreasing the mechanical spring constant K_(m), the initial position of the piston 6 is set to be closer to the TDC according to decrease of the mechanical spring constant K_(m), so that the piston 6 can completely reach the TDC.

The operation of the linear compressor in accordance with the present invention will now be explained.

First, when the current is applied to the coil wound body 14 a, the permanent magnet 16 is linearly reciprocated by interactions between the electromagnetic force generated at the periphery of the coil wound body 14 a and the permanent magnet 16, and the piston 6 connected to the permanent magnet 16 by the connection member 17 is linearly reciprocated inside the cylinder 4. As the piston 6 is linearly reciprocated inside the cylinder 4, the compression space P in the cylinder 4 is changed, and the refrigerants are sucked into the compression space P, compressed and discharged.

In detail, when the piston 6 is transferred in the direction of expanding the compression space P inside the cylinder 4, as illustrated in FIG. 7A, the inside pressure of the compression space P is reduced lower than a predetermined suction pressure, to open the suction valve 22. The refrigerants sucked through the inlet tube 2 a are sucked into the compression space P via the refrigerant passage 6 a of the piston 6.

Thereafter, when the piston 6 is transferred in the direction of compressing the compression space P inside the cylinder 4, as shown in FIG. 7B, the inside pressure of the compression space P increases in the close state of the suction valve 22 and the discharge valve 24 b, and thus the refrigerants are compressed into high temperature high pressure gas refrigerants.

In the case that the piston 6 is transferred in the direction of compressing the compression space P inside the cylinder 4 to reach the TDC, as depicted in FIG. 7C, the inside pressure of the compression space P is higher than a predetermined discharge pressure. Accordingly, the valve spring 24 c is compressed to open the discharge valve 24 b, and the refrigerants compressed in the compression space P are externally discharged through the loop pipe 28 and the outlet tube 2 b via the discharge space.

The linear compressor compresses the refrigerants by repeating the above procedure. The linear compressor performs the operation in the resonance state to improve efficiency, by synchronizing the operation frequency f_(c) of the linear motor 10 with the natural frequency f_(n) of the piston 6 calculated in consideration of the gas spring constant K_(g) varied by the load. In addition, the linear compressor varies the compression capacity by controlling the stroke S of the piston 6 by adjusting the current supplied to the linear motor 10 according to increase of the load, thereby rapidly handling the load and remarkably reducing power consumption.

As discussed earlier, when the mechanical spring constant is set lower than the general mechanical spring constant, the gas spring has greater influences than the general gas spring. In accordance with the present invention, as the influences of the gas spring increase, when the load increases, the natural frequency of the piston automatically increases.

The natural frequency of the piston is remarkably varied by the load, and the operation frequency of the linear motor is easily synchronized with the natural frequency of the piston. As a result, the linear motor is operated in the resonance state to maximize efficiency and rapidly overcome the load. Furthermore, the operation in the low frequency area reduces power consumption.

In addition, the stroke of the piston is controlled by adjusting the external current applied to the linear motor, thereby actively handling and rapidly overcoming the load and reducing power consumption.

The linear compressor in which the moving magnet type linear motor is operated and the piston connected to the linear motor is linearly reciprocated inside the cylinder to suck, compress and discharge the refrigerants has been explained in detail on the basis of the preferred embodiments and accompanying drawings. However, although the preferred embodiments of the present invention have been described, it is understood that the present invention should not be limited to these preferred embodiments but various changes and modifications can be made by one skilled in the art within the spirit and scope of the present invention as hereinafter claimed. 

1. A linear compressor, comprising: a fixed member having a compression space inside; a movable member linearly reciprocated in the fixed member in the axial direction, for sucking refrigerants into the compression space and compressing the refrigerants; one or more springs installed to elastically support the movable member in the motion direction of the movable member, spring constants of which being varied by load; and a linear motor installed to be connected to the movable member, for linearly reciprocating the movable member in the axial direction, and synchronizing its operation frequency with a natural frequency of the movable member dependent upon the spring constants.
 2. The linear compressor of claim 1, wherein the spring constants of the springs are varied in proportion to the load, and the operation frequency of the linear motor is varied in proportion to the load.
 3. The linear compressor of claim 2, which is installed in a refrigeration/air conditioning cycle, wherein the load is calculated in proportion to a difference between a pressure of condensing refrigerants (condensing pressure) and a pressure of evaporating refrigerants (evaporating pressure) in the refrigeration/air conditioning cycle.
 4. The linear compressor of claim 3, wherein the load is additionally calculated in proportion to a pressure that is an average of the condensing pressure and the evaporating pressure (average pressure).
 5. The linear compressor of any one of claims 1 to 3, wherein the springs comprise: a mechanical spring being installed to support the movable member at both sides of the motion direction of the movable member, and having a constant mechanical spring constant; and a gas spring having a gas spring constant varied by the load of the refrigerants sucked into the compression space.
 6. The linear compressor of claim 5, wherein the mechanical spring and the gas spring are formed so that the ratio of the mechanical spring constant to the total spring constant obtained by adding up the mechanical spring constant and the gas spring constant can be below 90%.
 7. The linear compressor of claim 5, wherein the mechanical spring constant and the gas spring constant of the mechanical spring and the gas spring are determined so that the natural frequency of the movable member can be set in a low frequency area between 30 and 55 Hz.
 8. The linear compressor of claim 5, wherein the linear motor varies a stroke that is a linear reciprocation distance of the movable member by the load.
 9. The linear compressor of claim 8, wherein the linear motor linearly reciprocates the movable member to reach a top dead center even if the stroke of the movable member is varied.
 10. The linear compressor of claim 9, wherein an initial position of the movable member is closer to the top dead center according to decrease of the mechanical spring constant.
 11. A linear compressor, comprising: a fixed member having a compression space inside; a movable member linearly reciprocated in the fixed member in the axial direction, for compressing refrigerants sucked into the compression space; a mechanical spring being installed to elastically support the movable member at both sides of the motion direction of the movable member, and having a constant mechanical spring constant; a gas spring having a gas spring constant varied by load of the refrigerants sucked into the compression space; and a linear motor installed to be connected to the movable member, for linearly reciprocating the movable member in the axial direction, wherein the mechanical spring constant and the gas spring constant of the mechanical spring and the gas spring are set so that a stroke that is a linear reciprocation distance of the movable member can be varied by the load.
 12. The linear compressor of claim 11, which is installed in a refrigeration/air conditioning cycle, wherein the load is calculated in proportion to a difference between a pressure of condensing refrigerants (condensing pressure) and a pressure of evaporating refrigerants (evaporating pressure) in the refrigeration/air conditioning cycle.
 13. The linear compressor of claim 12, wherein the load is additionally calculated in proportion to a pressure that is an average of the condensing pressure and the evaporating pressure (average pressure).
 14. The linear compressor of claim 11, wherein the mechanical spring constant and the gas spring constant of the mechanical spring and the gas spring are set so that the movable member can be linearly reciprocated to reach a top dead center even if the stroke of the movable member is varied.
 15. The linear compressor of claim 14, wherein an initial position of the movable member is closer to the top dead center according to decrease of the mechanical spring constant. 